Certain television broadcast services, such as the Multipoint Distribution Service, also known as MDS, have created a requirement for reliable inexpensive microwave antennas. The antennas must meet rather demanding requirements. At the present time, MDS stations under FCC regulations transmit with only 10 watts of power and, accordingly, a high gain antenna is desirable. Receiving locations are often at single occupancy dwellings, which requires that for reasons of economic feasibility the cost of installation be minimal. Reliability is important to avoid the cost of service calls. Low wind load and ease of installation are other factors.
I have found that the disc-rod antennas of the general type disclosed in my prior U.S. Pat. No. 2,955,287 issued on Oct. 4, 1960, may be utilized in supplying the present requirement. However, as will be explained hereinafter, only a narrow range of the previously indicated useful range of disc size and spacing may be utilized for the purposes of the present invention.
The disc-on-rod type antenna, excited by a launcher or elementary antenna as described in the above-referenced patent, can take the form of uniformly spaced discs or plates of uniform size and shape. In general, the design as described in the above-referenced patent required that to achieve optimum gain for a given element length (measured in wavelength) the plate size and spacing be selected for that particular length. Therefore, a length change to increase or reduce gain to meet system requirements was of necessity accompanied by either loss of optimum gain for the element length which is, in effect, a less efficient design, or require a change of plate size and/or spacing. I have found that there is a design, however, which is such that the longer the element the higher the gain at a given frequency, with no requirement to change plate size or spacing, but only length, to change gain and remain within a few percent of optimum gain for the element length.
Thus, use of a simple launcher, as e.g. a probe-excited open end wave guide or pair of dipoles, in conjunction with a simple mount to a mast which allows either horizontal or vertical polarization and pointing change in azimuth and elevation, and in conjunction with a disc-on-rod designed to be optimum over a large range of length with the same plate dimension and spacing throughout, can allow an installer to use no more than the required disc-on-rod length for each location. The length may, in practice, be varied in many simple ways, such as adding length in steps, replacement with units of different length, or actually adding or removing plates individually.
I have found that the plate shape, size, and spacing which are optimum for this purpose are: plates which are round thin flat discs normal to the axis, of constant diameter D where (D-d) is in the range of 0.23.lambda. to 0.27.lambda., and of spacing between 0.18.lambda. and 0.23.lambda. (where d is the diameter of the conductive support). In this range the gain can be made to be within a few percent of optimum as a function of length for a range of lengths 3.lambda. to 12.lambda. long. The disc-on-rod having these parameters provides a gain of 4 db above the launcher gain to 9 db above the launcher gain. The wavelength, .lambda., is the wavelength of the signal of interest.
The above brief description, as well as further objects, features and advantages of the present invention, will be more fully appreciated by reference to the following detailed description of a presently preferred, but nonetheless illustrative, embodiment in accordance with the present invention when taken in conjunction with the accompanying drawings.